The present invention is directed generally to alternating current power generation and more particularly to frequency regulators and protective devices for alternating current generators.
With increasing interest in energy production, it is desirable to provide a relatively inexpensive and simple electrical power generating apparatus that can be driven from power sources that were previously thought to be unsuitable. For example, and with particular reference to hydroelectric power generation, water flows can be found in mountainous regions or in releases from reservoirs and water supply systems which have relatively low flow rates with heads in the range of two hundred to six hundred feet.
When generating alternating current, the input mechanical power applied to the generator must substantially equal the output electrical power provided by the generator to an electrical load in order to stabilize the frequency of the alternating current. Moreover, this alternating current frequency must be stabilized at a frequency suitable for the particular electrical load connected to the generator which is commonly 60 Hz in the United States.
For example, the water flow rate through the prime mover of a hydroelectric generator can be controlled to thereby control the generator output frequency. This is typically accomplished by means of modulating valves or gates which are adjusted by relatively complicated speed governors. These water flow rate control systems are generally costly and often require continous maintenance. Moreover, sudden changes in the modulating valves or gates can cause undesirable and serious pressure transients in the water supply conduits feeding the prime mover when the water flow rates are suddenly changed. Since the flow rate in the system may be substantially changed or even completely cutoff by the control system, such as system is not practical in applications such as releases from reservoirs in water supply systems as described above.
It is known that for a constant input power to the generator, the frequency of the generator can be maintained at a given value by maintaining a constant load. A local load connected to the generator output is then adjusted to compensate for changes in the remaining electrical load. Such systems are disclosed, for example, in U.S. Pat. Nos. 2,015,556 to Fountain and 3,538,391 to Bensley et al. These systems are constant load systems and thus if the input power varies from the initial constant value, the generator output frequency correspondingly varies. Such systems are therefore not suitable where the power supplied to the generator prime mover can vary as would be the case, for example, with a water supply system where the reservoir water flow release would be changed to respond to demand changes within the water system or in regions where the water level in the reservoir varies. Additionally, systems such as Bensely et al are current regulators and are not true load regulators since the generator output current is not necessarily a true measure of load or power as would occur with, for example, reactive external loads.
Commercially available devices can be used for protecting a generator from over current operation or current imbalance between phases of a polyphase alternating current generator. Typically these devices are electromechanical in nature, are relatively expensive and physically large, respond relatively slowly to changing current conditions in the generator, and require protection from the environment in which the devices operate. It is thus desirable to provide faster, lower cost devices that withstand environmental extremes not suitable for electromechanical devices.